/Engine/Plugins/Runtime/Metasound/Source/MetasoundEngine/Public/MetasoundGeneratorHandle.h
UMetasoundGeneratorHandleshields its "clients" from "cross thread" issues related to callbacks coming in the audio control or rendering threads that game thread clients (e.g. blueprints) want to know about. That is why these next delegate definitions do not need to be the "TS" variants. Assignments to members of this type, and the broadcasts there to will all happen on the game thread. EVEN IF the instigator of those callbacks is on the audio render thread.
typedef TMulticastDelegate_NoParams< void > FOnAttached